The present invention relates to the field of heteropoly acids, and more particularly those heteropoly acids which comprise vanadium, tungsten, phosphorous and/or molybdenum.
Heteropolyacids are a recognized class of acids containing large amounts of oxygen and hydrogen, and multiple atoms of one or more elements, such as molybdenum or tungsten, surrounding one or more heteroatoms of another element, such as phosphorous. Polyanions of such acids consist primarily of octahedral MoO.sub.6 or WO.sub.6 groups, so that the conversion of [MoO.sub.4 ].sup.2- or [WO.sub.4 ].sup.2- into polyanions requires an increase in coordination number. Cotton and Wilkinson, "Advanced Inorganic Chemistry", 4th ed., pp. 852-861, Wiley & Sons, N.Y. (1980) disclose that heteropolyanions can be formed either by acidification of solutions containing the requisite simple anions, or by introduction of the hetero element after first acidifying the molybdate or tungstate. As indicated at Table 22-C-2 of Cotton and Wilkinson (pg. 857), various heteropolyanion formula types are known.
Heteropolyacids such as molybdophosphoric acids, are known to exist in the stoichiometry of the "Keggin" structure (PMo.sub.12 O.sub.40.sup.3-), as well as in the stoichiometry of a Dawson structure (P.sub.2 Mo.sub.18 O.sub.62.sup.6-). Of these structures, the "Keggin" structure is the most commonly formed cluster, and Keggin structure molybdophosphoric acids are known to be suitable vapor phase catalysts in the oxidative dehydrogenation of isobutyric acid to methacrylic acid. In Japanese Patent Disclosure No. 1975-4014 dated Jan. 16, 1975 entitled "A Process for Manufacturing Methacrylic Acid, abstracted at Chemical Abstracts, Volume 83, 4408b (1975), the use of molybdophosphoric acid having the empirical formula H.sub.3 Mo.sub.12 PO.sub.40.nH.sub.20, as well as molybdovanadophosphoric acid, are disclosed for use in vapor phase oxidative dehydrogenations of isobutyric acid. Such reactions are performed in the presence of oxygen and other gases such as nitrogen, steam, etc., such reactions being carried out in the temperature range of 200.degree.-400.degree. C., preferably 250.degree.-350.degree. C. More particularly, this Japanese patent disclosure indicates that the selectivity of methacrylic acid can be improved by using a catalyst which is prepared by adding a sulfate of an alkali metal, nickel or cobalt to a heteropolyacid.
It has long been known to use various heteropolyacids to catalyst certain organic reactions. For example, in U.S. Pat. No. 4,192,951, vapor phase oxidation procedures are disclosed utilizing various heteropolyacid catalysts, including heteropolymolybdic catalysts containing vanadium, tungsten, tantalum or niobium. Such compounds act as catalysts for the synthesis of materials such as maleic acid and acetic acid. U.S. Pat. No. 4,192,951, also discloses a molybdophosphoric acid catalyst having an empirical formula of H.sub.6 [P.sub.2 Mo.sub.18 O.sub.62 ] which was prepared using a procedure involving the refluxing of Mo.sub.3 and H.sub.3 PO.sub.4 overnight to produce a bright yellow filtrate. Although the empirical formula provided relating to the molybdophosphoric acid catalyst of the '951 disclosure corresponds to the empirical formula of a Dawson structure catalyst, no mention is made in the '951 patent of the stoichiometry of the structure obtained in Example 1. It is clear from the filtrate color reported in the '951 patent that the stoichiometry of the '951 catalyst is not of the "Dawson" type. In an article entitled "Contribution To The Chemistry of Phosphomolybdic Acids, Phosphotungstic Acids, and Allied Substances", by Hsein Wu, J. Biol. Chem., 43, 189 (1920) a proper procedure for preparing phospho-18-molybdic acid of the Dawson structure is disclosed. As explained by Wu at pages 196 and 197, care must be taken during the preparation of such an acid to avoid the formation of yellow crystals and to obtain orange crystals which are indicative of phospho-18-molybdic acid of the Dawson structure.
One approach to the preparation of heteropolyanions is the formation of compounds with a Keggin-defect structure which are derived from an alpha-PM.sub.12 structure by removing one MO.sub.6 octahedron or three MO.sub.6 octahedra of the same (M.sub.3) set. See "P-NMR Studies on Molybdic and Tungstic Heteropolyanions: Correlation Between Structure and Chemical Shift", Massart et al, Inorganic Chemistry, 16, 2916-2921 (1977). Massart discloses that metal atoms, other than tungsten can partly or wholly fill these holes, giving rise to such compounds as PW.sub.9 Mo.sub.3, PW.sub.10 Mo.sub.2, or PW.sub.11 Mo. In particular, Massart discloses the synthesis of various molybdotungstophosphoric acids wherein holes in the defect structure are filled with atoms of a given metal, such as molybdenum. As a result, Massart discloses compounds having up to two transition metals in a heteropolyanion structure.
In U.S. Pat. No. 4,146,574 (Onoda et al) entitled "Process for Preparing Heteropoly-acids", certain heteropolyacid catalysts are disclosed as being useful in oxidations and oxidative dehydrogenations, as for example, the oxidative dehydrogenation of isobutyric acid to methacrylic acid, and the oxidative dehydrogenation of isobutyraldehyde to methacrolein and methacrylic acid. (See columns 7 and 8, and particularly Tables III and V). The heteropolyacids of U.S. Pat. No. 4,146,574 are indicated as being represented by the general formula: EQU H.sub.3+x Mo.sub.12-x-y W.sub.y V.sub.x PO.sub.40.nH.sub.2 O
wherein x is an integer of 1 to 4, y is an interger of 0 to 3, the sum of x+y is 1 to 4, and n represents a number for the water of crystallization and usually has a value within the range of 16 to 32 in the crystalline state. Although this formula may be considered to disclose a vanadotungstomolybdophosphoric heteropoly acid, U.S. Pat. No. 4,146,574 provides no example of such a heteropolyacid. Further, as described hereinafter, applicants have found that the synthetic procedure set forth in the Onoda patent will not result in the incorporation of tungsten in a vanadomolybdophosphoric acid.
While the above-described methods for converting isobutyric acid to methacrylic acid have achieved some success, a need still exists for methods for efficiently and selectively converting isobutyric acid to methacrylic acid.